It is well known that radio waves, propagating from a transmitter to a receiver, can follow a plurality of different paths, and that the relative phase of the different waves arriving at the receiving antenna can be such as to destructively interfere, causing what is commonly referred to as a fade. In order to protect against this condition, the so-called "space diversity" system has been developed using two, spaced antennas to feed a diversity receiver which drives a common detector. The theory underlying the use of two spaced-apart antennas is that there is less likelihood that a fade will occur at both antennas at the same time. In the simplest system, means are provided to disconnect the receiver from one antenna as soon as the received signal level falls below a predetermined threshold and to connect the receiver to the second antenna. In this co-called "blind switching," it is assumed that the signal received by the second antenna is stronger than that received by the first antenna. In a more sophisticated system, the signals from the two antennas are combined at radio frequency or at an intermediate frequency instead of switching between the two. This eliminates amplitude and phase jumps associated with the switching operation, and has the added advantage of delivering a larger amplitude signal to the receiver.
In analogue radio systems, it is the practice to adjust the phases of the two received signals so as to maximize the total power in the combined signal. For example, in U.S. Pat. No. 2,786,133 the relative phase of the local oscillator signals coupled to a pair of frequency converters is adjusted such that the resulting IF signals produced combine to maximize the total signal.
Studies have shown, however, that the situation is more complex in a digital radio system. In particular, it has been found that the bit error rate (BER) in a digital system is more sensitive to the degree of inband amplitude dispersion and less sensitive to the magnitude of a flat fade. Accordingly, the Nippon Electric Company (NEC) includes in their Model RP-6G178MB, 78Mb/s, 6GHz, 8PSK system an equalizer following their maximum power signal combiner. The Collins Radio Division of Rockwell International provides in their Model MDR-6 radio system two separate, equalized receivers and switches between them at baseband as signal conditions require.
The broad objective of the present invention is to simplify space diversity receivers used in digital radio systems.